A glow lamp typically is comprised of a light transmitting envelope containing a noble gas and mercury with a phosphor coating on an inner surface of the envelope which is adapted to emit visible light upon absorption of ultraviolet radiation that occurs when the lamp is excited. The lamp is excited by means of the application of a voltage between the lamp electrodes. Current flows between the electrodes after a certain potential is applied to the electrodes, commonly referred to as the breakdown voltage. An elementary explanation of the phenomenon is that the gas between the electrodes becomes ionized at a certain voltage, conducts current, and emits ultraviolet radiation. Examples of typical glow discharge lamps are found in U.S. Pat. Nos. 2,067,129 to Marden; 3,814,971 to Bhattacharya; and 4,408,141 to Byszewski, et al.
Reference is also now made herein to a co-pending application Ser. No. 139,397 filed concurrently herewith on a DC operated negative glow discharge lamp employing a cathode coated with an emissive material and a bare anode. FIG. 1 herein illustrates a glow discharge lamp of this type including an envelope 10 that is provided with a phosphor coating as illustrated at 12. There may be one or more electron emitting electrodes (cathodes) and one or more electron collecting electrodes (anodes). FIG. 1 in particular illustrates a cathode electrode 14 and an anode electrode 16. These electrodes are supported by respective lead-in wires 15 and 17.
In FIG. 1 the envelope 10 is generally of spherical shape having a generally maximum cross-section bulbous region 18 and also including a neck region 20. The lead-in wires 15 and 17 are typically hermetically sealed at the neck region 20 with a wafer stem assembly. In FIG. 1 the electrodes 14 and 16 are supported primarily in a side-by-side relationship and are approximately at the maximum cross-section bulbous region 18.
In operation, the cathode emits electrons that are accelerated so that mercury vapor is excited in the extended region of the low pressure gas. In this connection the envelope may be filled with a conventional fill material including mercury and a noble gas or mixtures of noble gases. A suitable noble gas is neon. Furthermore, the lamp can be operated from either an AC or DC power source.
In the glow discharge lamp described in co-pending Application Ser. No. 139,397 filed concurrently herewith, the cathode electrode is coated with an emissive material while the anode electrode is uncoated. The anode electrode is typically a bare tungsten coil electrode. The lamp is operated in a DC mode of operation rather than an AC mode of operation. This lamp construction provides improved lumen maintenance and longer life span, particularly in comparison to prior art glow lamp constructions. However, a tungsten electrode, devoid of an emissive coating while serving as an anode, in a DC operated lamp, can have certain drawbacks associated therewith. Generally, two lead-in wires are required to support the tungsten electrode. Furthermore, the tungsten electrode is costly, particularly in comparison to the cost of the lead-in wires. Moreover, the tungsten electrode can cause a power loss in the lamp circuit due to IR heating of the electrode during lamp operation.